The present invention relates to a flexible pipe for transporting, over long distances, a fluid which is under pressure and possibly at a high temperature, such as a gas, petroleum, water or other fluids. The invention relates most particularly to a pipe intended for offshore oil exploration. It relates especially first, to the flow lines, that is to say flexible pipes unwound from a barge in order to be laid generally on the bottom of the sea and connected to the subsea installations, such pipes working mainly in static flexible pipes which are unwound from a surface installations and most of which do not lie below the seabed, such pipes working essentially in dynamic mode.
The flexible pipes used offshore must be able to resist high internal pressures and/or external pressures and also withstand longitudinal bending or twisting without the risk of being ruptured.
They have various configurations depending on their precise use but in general they satisfy the constructional criteria defined in particular in the standards API 17 B and API 17 J drawn up by the American Petroleum Institute under the tile "Recommended Practice for Flexible Pipe". Reference may also be made to documents FR 2 654 795 A, WO 938/25 063 A, FR 2 727 738 A and FR 2 744 511 A.
A flexible pipe generally comprises, from the inside outward:
an internal sealing sheath made of a plastic, generally a polymer, able to resist to a greater or lesser extent the chemical action of the fluid to be transported; PA1 a pressure vault resistant mainly to the pressure developed by the fluid in the sealing sheath and consisting of the winding of one or more interlocked profiled metal wires (which may or may not be self-interlockable) wound in a helix with a short pitch (i.e. with a wind angle close to 90.degree.) around the internal sheath; PA1 at least one ply (and generally at least two crossed plies) of tensile armor layers whose lay angle measured along the longitudinal axis of the pipe is less than 55.degree.; and PA1 an external protective sealing sheath made of a polymer.
Such a structure is that of a pipe with a so-called smooth bore. In a pipe with a so-called rough bore, a carcass consisting of an interlocked metal strip, which serves to prevent the pipe being crushed under external pressure, is also provided inside the internal sealing sheath. However, the pressure vault also contributes to the crushing strength.
Attempts are made to reduce the weight of flexible pipes, particularly for applications at great depth, where, in order to rests being crushed, it is necessary to considerably increase the moment of inertia of the profiled wire constituting the pressure vault. The weight of the flexible pipe also plays an important role when laying it; this is because its weight must be limited so as to allow it no be laid by existing means (for example 600 tonnes for a conventional system).
The pressure vault consists of a profiled wire, usually of the Z or T type, or derivatives (teta and zeta) thereof, which is wound with a short pitch. The profiled wire is generally such that the ratio of its height to its width is less than 1, so as to prevent warping in winding the vault. In addition, it is known to dimension the pressure vault so that it helps to delay the onset of ovalization of the carcass under the increase in internal pressure (this onset resulting in the ruin of the carcass), but promotes the extension of the preferred cardioidal deformation mode: the delay in ovalization is all the greater the higher the moment of inertia I.sub.xx, of the profiled wire constituting the vault.
For applications at great depth, it is therefore desired to increase the moment of inertia of the profiled wire usually employed, in order to resist the crushing pressure; for example, it would be desirable to use a teta wire 16 mm in height. However, this would result in drawbacks, such as the increase in the weight of the pipe which may exceed the limit of the laying system, or even exceed the limits of resistance of the pipe itself being able to support its own weight when laying it; and a more complex implementation of this type of profiled wire; all these drawbacks increase the manufacturing cost of such a pipe.
The oil industry is therefore seeking an interlockable profiled wire having a high moment of inertia I.sub.xx for a low weight.
It has already been proposed, in document U.S. Pat. No. 4,549,581 A, to use interlockable U-shaped profiled wires, but the improvement made to the moment of inertia/weight ratio has not been significant. Moreover, it appears not to be easy to envision lightening the known S-, Z- or T-shaped sections by providing hollows, for manufacturing reasons.